A microgrid (MG) is a small-scale power network that operates in grid-tied mode under normal operating condition and switches to islanded mode when grid disturbance occurs. The transition from the grid-tied mode to the standalone mode is known as the islanding process. For fault-induced islanding, voltage in the MG can drop to as low as 0.2 per-unit (p.u.), and it usually takes more than 30 cycles for the voltage to recover to its nominal value. This voltage recovery process may take much longer if the MG is heavily penetrated with dynamic load, such as single-phase induction motors (SPIMs). These motors stall under low voltage condition (e.g., less than about 0.87 p.u.), after which they absorb two to three times the rated power, making power generation in the MG insufficient. Some predictions put the eventual penetration of SPIMs in a distribution system as high as 75% due to government incentives and energy efficiency requirements. It is dangerous to keep voltage in an MG low for a long period, since load will be shed by under-voltage load shedding protection schemes. Using reactive power generated from distributed energy resources (DERs) is a possible solution for MG voltage regulation.
Two existing methods for sharing reactive power among electronic-interfaced distributed generators (EIDGs) have been proposed. A first is to use droop control to solve the reactive power sharing problem among distributed generators (DGs). By drooping voltage references of DG controllers against the real or reactive power outputs, parallel operation of DGs is enabled. However, droop control is based on local voltage measurements only and is incapable of regulating the voltage at buses that have no DG nearby.
Another method is to use a method based on voltage sensitivity to regulate the voltage of a specific bus. By adjusting the reactive power output of a DG, e.g., a wind generator, based on its active power generation, voltage at the targeted bus is constrained to a certain limit. This method may work well when the MG is operating at steady state with only one DG, but it does not take into account the MG under transients or with multiple DGs.
As such, existing methods of controlling reactive power generation and properly sharing the burden among EIDGs are inadequate to solve the problem.